1. Field of the Invention
The present invention relates generally to subterranean formation permeability contrast correction methods, and more particularly, to methods for forming in situ gels in water flooded subterranean formations to plug high permeability zones therein.
2. Description of the Prior Art
In water flooding operations, injected flood water flows from one or more injection wells through the subterranean formation being flooded to one or more production wells. The flood water flows through zones of high permeability to the producing well bores whereby oil in such high permeability zones is recovered. Because the high permeability zones take most of the injected flood water, low permeability zones and the oil contained therein are generally bypassed. While this is acceptable initially when the oil contained in the high permeability zones is being swept therefrom, it is subsequently undesirable as the oil in the high permeability zones becomes depleted. From that point on, the water flood provides little benefit in enhancing oil production.
Methods and compositions for accomplishing near well bore and in depth plugging of high permeability zones in subterranean formations by forming gels therein have heretofore been developed and used. The formation of the gel in high permeability zones causes the zones to be plugged or at least lowered in permeability whereby subsequently injected flood water is caused to enter previously bypassed low permeability zones. This in turn causes the flood water to mobilize increased amounts of oil which are recovered from the subterranean formation.
A variety of methods and polymer compositions which gel in situ to reduce the permeability of high permeability zones in subterranean formations have been developed and used heretofore. For example, U.S. Pat. No. 3,762,476 issued on Oct. 2, 1973 discloses a method of reducing recovered water from a subterranean formation wherein aqueous polymer solutions interspaced with aqueous solutions of crosslinking metallic ions are injected into the formation. A variety of polymers can be utilized in conjunction with various multivalent metal cations which cause the polymers to be crosslinked.
U.S. Pat. No. 4,552,217 issued on Nov. 12, 1985 discloses a water permeability correction process to improve the sweep efficiency of water flooding which involves the sequential injection of an aqueous solution of a crosslinking agent, i.e., sequestered polyvalent metal cations and an aqueous solution containing a crosslinkable polymer such as polyacrylamide.
U.S. Pat. No. 4,683,949 issued on Aug. 4, 1987 describes a conformance improvement method using a crosslinkable gel comprised of a polyacrylamide polymer and a chromium III acetate crosslinking agent in an aqueous solvent.
While the above described methods and compositions as well as other methods and compositions have been utilized successfully for improving the sweep efficiency of water flooding, such methods and compositions have generally been unsuccessful in subterranean formations containing high permeability zones lying adjacent low permeability zones wherein fluid communication takes place at the boundaries between the zones. In such subterranean formations, the near well bore and/or in depth plugging of the high permeability zones does not confine flood water to the low permeability zones because of the flood water communication between the high permeability and low permeability zones.
Thus, there is a need for improved permeability contrast correction methods to improve the sweep efficiency of water flooding operations carried out in subterranean formations containing high permeability zones lying adjacent to low permeability zones wherein there is fluid communication between the zones at the boundaries therebetween.